Technical Field
The present invention relates to a cryopump and a method of operating the cryopump.
Description of Related Art
When a new cryopump is installed on site, the cryopump is cooled from a room temperature to a cryogenic temperature and a vacuum pumping operation is initiated. Further, the cryopump is a gas entrapment vacuum pump, as known, and hence regeneration is performed at a certain interval in order to discharge entrapped gas to the outside. Regeneration processing generally includes a temperature-raising step, a discharge step, and a cooling step. When the cooling step is terminated, the vacuum pumping operation of the cryopump is resumed. The cooling of the cryopump, performed as preparation for such a vacuum pumping operation, may be sometimes referred to as cool-down.
Although the cryopump is one of the major applications of a cryogenic refrigerator, it is different from other applications in that a relatively large temperature difference is required between a high-temperature stage and a low-temperature stage of the refrigerator. However, when the cryopump is cooled, it is not easy to create such a temperature difference in a short period of time. For example, if the temperature of the low-temperature stage does not yet reach its target temperature when the temperature of the high-temperature stage reaches its target cooling temperature, the cooling of the low-temperature stage is required to be still continued, while the high-temperature stage is being maintained at the target temperature. Alternatively, the high-temperature stage can already be cooled to a temperature lower than the target temperature, when the temperature of the low-temperature stage reaches the target temperature. In this case, the temperature of the high-temperature stage is required to be raised to the target temperature. Such temperature adjustment in the end of the cool-down takes a certain period of time. In particular, when a large temperature difference is required between the high-temperature stage and the low-temperature stage, the temperature adjustment takes a long period of time. Since the cool-down is a downtime of the cryopump, it is desirable to carry out the cool down in a short period of time.